You can upload a photo to your profile here: https://garden.org/users/prefs...
. Near the bottom of the page there should be a section marked "Avatar Photo".
Regarding adding your photos to the database, first find the entry for the plant you wish to add a photo for. There should be a box on the left with "Adding Actions" at the top. Click "Upload a photo" and follow the prompts.
You can also associate a photo that has already been uploaded into a post by clicking on the photo (to show the full view), then clicking on the little database icon (the circular platters) below the photo. This will take you to a search page where you can search for and select the appropriate entry.